Known, for instance, is a linear actuator for an electric parking brake drive apparatus, etc., which reduces a load on a driver by employing motive power of an electric motor when operating a parking brake of a vehicle. This type of linear actuator is proposed, wherein a parking brake is operated by wining up a cable in a way that rotates a pulley with an electric motor, and is canceled by rewinding the cable (refer to Japanese Patent Application Laid-Open Publication No. 2001-106060).
In the linear actuator disclosed in the aforementioned Publication, the motive power from the electric motor is decelerated and thus transmitted to a parking brake by use of a worm gear and a worm wheel. The power transmission conducted by employing the worm gear and the worm wheel has such a merit that a speed reduction ratio can be taken large, and the motive power from the parking brake can be prevented from being transmitted to the electric motor side by properly designing a torsional angle of the worm.
On a reversed side of this merit, however, the worm gear and the worm wheel have a demerit that the transmission efficiency is low. Therefore, an output of the electric motor must be increased for operating the parking brake, resulting in such a problem that the electric motor increases in size and is unable to scheme to save the electric power. Further, in the case of using the linear actuator other than the parking brake apparatus, some have a moment force generated at a movable portion, and there arises a problem about how to sustain this.